The present invention relates in general to communication systems and components therefor, and is particularly directed to a highly efficient, dual pulse width modulated driver for controlling the operation of a thermo-electric cooler, such as may be used for stabilizing the operating temperature of a laser employed in a digital optical transmitter for a very high data rate (e.g., on the order of 10 Gb/s or higher) communication system.
In order to maintain wavelength stability and reliability, as well as comply with intended performance specification of a component, such as a laser employed in a very high data rate (e.g., on the order of 10 Gb/s or higher) digital optical transmitter, it is common practice to thermally couple the laser with an associated thermo-electric cooling unit. Conventional techniques for controlling the current through and thereby the cooling/heating of the component provided the thermoelectric cooler include the use of xe2x80x98Hxe2x80x99-switched bridge circuits and extremely linear (and therefore costly) voltage sources. Unfortunately, at best, these prior art schemes are capable of achieving an efficiency of only 47%, which results in substantial power dissipation in the laser transmitter""s heat sink, particularly at high temperature operation. For an illustration of non-limiting examples of patent literature describing prior art control circuits for thermo-electric coolers, attention may be directed to the following U.S. Pat. Nos.: 5,602,860; 5,088,098; 4,631,728; 5,450,727; 4,792,957; and 5,118,964.
Pursuant to the present invention, rather than having to dissipate wasted power in the laser driver circuitry, thermo-electric cooler operation is controlled by using a class D-type device, in particular, a pulse width modulator-based driver arrangement that has an efficiency approaching 100% and generates a signal waveform having a duty cycle that is linearly proportional to a control voltage. This signal waveform is coupled to an interface circuit that is coupled to a power supply for operating the thermoelectric cooler, and is operative to apply a differential powering voltage across drive inputs of the thermo-electric cooling element in linear proportion to the duty cycle of the signal waveform generated by the pulse width modulator, so that the thermal characteristic of the thermo-electric cooling element is linearly proportional to the duty cycle of the signal waveform.
In accordance with a non-limiting but preferred embodiment, the pulse width modulator has complementary outputs coupled to control inputs of dual pairs of complementary polarity electronic switching elements, such as pairs of opposite polarity channel field effect transistors, that are coupled in circuit between first and second power supply terminals and a pair of complementary output nodes. The complementary output nodes are coupled in common through a low pass filters to opposite powering inputs of the thermo-electric cooling element. The thermal output generated by the thermoelectric cooling element is coupled to a temperature stabilized device (e.g., laser), and is monitored by a temperature sensing element, such as a thermistor mounted on the surface of the device (laser). The thermistor provides a thermal sensor output voltage in linear proportion of its thermal input, which is fed back through a temperature control servo loop to a differential summing circuit, and combined with a reference voltage used to control the duty cycle of the pulse width modulator. The output of the differential summing circuit is coupled to the modulator through a low pass loop filter to eliminate the modulator""s switching frequency component in the fed back sensor output voltage from the thermistor. In operation, the pulse width modulator operates as a class D-type driver, generating a pair of complementary switching signal waveforms in linear proportion to its DC input voltage. For a 50% duty cycle, the voltage levels of the complementary switching waveforms will be evenly split for each cycle, so that each of the thermo-electric cooling element""s drive inputs will see the same DC voltage, resulting in a zero DC volt differential across the thermo-electric cooling element, and a corresponding zero drive current being imparted to the thermo-electric cooler. This differentially based zero drive eliminates the xe2x80x98dead zonexe2x80x99 around zero current present with a conventional arrangement of a linear driver and a switch bridge, resulting from errors in current direction control circuitry.
For an input voltage to the pulse width modulator that changes the duty cycle from 50%, the duty cycle of one of the modulator""s signal waveform outputs will linearly change (e.g., increase) in a complementary manner relative to the linear change (e.g., decrease) in the duty cycle of its other (complementary) signal waveform output. This causes different percentages of the difference between the power supply rails to be applied to the respective drive inputs of the thermo-electric cooling element, and thereby creating a non-zero voltage differential across the thermo-electric cooler. Such complementary apportioning of the difference between the power supply rails as the drive inputs to the thermo-electric cooler makes control of the thermo-electric cooler effectively insensitive to fluctuations in the power supply voltages. Namely, the thermal output of the thermo-electric cooling element (heating or cooling) can be very accurately (and linearly) controlled in accordance with an increase or decrease in the duty cycle of the pulse width modulator.